Electric contact and the like



y 1943 E. l. LARSEN EI'AL 2,319,240

ELECTRIC CONTACT AND THE LIKE Filed March 19, 1940 INVEII TORS Ear! Z lamen avid/"IF Jzmzq ATTO R N EY May W, 1943 ELECTRIC CONTACT AND THE LIKEEarl l. llarsen and Earl F. Swazy, Indianapolis,

Ind., assignors to P. B. Mallory & 00., Inc., Indianapolis, Ind., acorporation of Delaware Application March 19, 1940, Serial No. 324,742

2 Claims. '(Ol. 75-22) This invention relates to facing elements such aselectric contacts and the manufacture thereof.

An object of the invention is to improve such elements and the methodsof manufacture thereof.

Other objects of the invention will be apparent from the followingdescription taken in connection with the appended claims.

The present invention comprises a combination of elements, methods ofmanufacture and the product hereinafter set forth, the scope of theinvention being indicated in the appended claims.

While a preferred embodiment of the invention is described herein, it iscontemplated that considerable variation may be made in the method ofprocedure and the combination of elements, without departing from thespirit of the invention.

In the drawing:

Figure 1 illustrates a contact disc during a preliminary stage of itsmanufacture;

Figure 2 is a cross section of the contact disc illustrated in Figure 1;

Figure 3 illustrates the furnacing or sintering of the contacts;

Figure d is a cross section of a contact after furnacing;

Figure 5 is a cross section of the contact after a subsequent pressingoperation;

Figure 6 is a perspective View of the repressed contact; and

Figure 7 illustrates a contact during a preliminary step in itsprocessing according to a modified procedure.

The present invention relates to improvements in face plates orsurfacing elements, particularly of the type formed of mixtures ofmetals and carbon or graphite particles, or other carbonaceous material.In its preferred embodiment the invention contemplates electric contactplates, or discs, formed of a mixture of powdered contact metal andgraphite, pressed into suitable form and sintered.

A feature of the invention resides in the improved structure andcomposition of the face plate or contact disc wherein carbon or graphiteis present in the metal composition at one face and substantially absentat the other face of the plate. Another aspect of the invention residesin the improved method of manufacture wherein the plate or disc is firstformed with graphite present throughout and the graphite is subsequentlyremoved from one surface by a degraphitizing process.

The metal used will depend upon the application intended. It is apparentthat any metal which is not substantially deleteriously affected by theprocess will be suitable. For contact purposes a metal having desirablecharacteristics for contacts is preferred, such as silver, copper,nickel, cadmium, tungsten and molybdenum or alloys or mixturescontaining one or more of these elements. The metal or alloy ispreferably mixed, in powder form, with the graphite or othercarbonaceous material. The preferred composition for contact purposes isa pressed and sintered mixture of silver and graphite powders.

One of the principal objections, heretofore, to

a contact composed of silver and graphite is the fact that it isextremely difficult to attach a backing member thereto. It can not beriveted, staked, screwed or otherwise mechanically attached to thebacking material in an efilcient manner. Therefore, it is necessary thatthe silver graphite pressed material be soldered or brazed to thesupporting or current carrying backing. It has been found that when theusual solders are used to perform this brazing operation, that a veryweak joint or bond between the silver graphite material and the backing,is obtained. This appears to be due to the fact that when the silvergraphite material is heated, the graphite is not tinned or wetted by thesoldering. Furthermore, it appears to be very gassy and does not allowthe silver to tin or solder properly. This objection or deficiency isentirely overcome when contacts of, for example, silver and graphite,are manufactured according to the present invention.

Following is a description of a, preferred method of preparing silvergraphite contacts according to the invention, referencebeing made to theaccompanying drawing:

A powder mixture comprising approximately by weight of silver powder and5% graphite powder is prepared by mixing the powder thoroughly. Themixed powders are then pressed into the desired shape, for example, thecontact disc ill illustrated in Figure 1 of the drawing.

The pieces are then laid on a graphite slab H- with one face directly incontact with the graphite slab. The graphite slab ll carrying thecontacts Ill is then placed in a heated chamber such as electric furnacel 2 of Figure 3, the chamber having an oxidizing atmosphere.

The cross section of the pressed contact discs, prior to sintering, mayappear somewhat as illustrated in Figure 2 of the drawing wherein thegraphite particles l3 are interspersed with the silver ll in which theremay also be present at this time a few voids IS.

The contact discs are allowed to remain in the heated chamber for a timesuiiicient to sinter the silver particles together and to degraphitizethe exposed surfaces of the contacts. With a furnace temperature of 900degrees C. a period of hour is generally satisfactory.

The degraphitizing, which comprises an important aspect of the presentinvention, is believed to take place in the following manner: Theoxidizing atmosphere, for example air, reacts with the graphiteparticles at and adjacent to the exposed surface of the contacts to formgaseous carbon monoxide or carbon dioxide or both, thereby denuding thepressed mixture of graphite adjacent to the surface. However, theopposite face of the contact which is resting against the graphite slabis protected from oxidation by the graphite slab and hence the graphiteparticles remain substantially undiminished in quantity at the lattersurface.

While air is a satisfactory oxidizing atmosphere, other oxidizingatmospheres may be used such as pure oxygen or even hydrogen, containinga small percentage of oxygen or water vapor. The water vapor reacts withthe graphite to form carbon monoxide with the release of pure hydrogen.

Upon removal from the furnace the contact disc may have a cross sectionsuch as illustrated in Figure 4. It will be noted that the graphiteparticles ii are removed from the surface I6 and for a substantial depthbelow the surface, while the surface I! which was in contact with thegraphite slab still contains graphite in substantially undiminishedquantity. It will also be noted that the contact disc l has shrunksomewhat in thickness. In order to increase the density of the contactdisc and substantially eliminate the voids it is generally preferred,although not essential, to apply a subsequent repressing or coiningoperation which further reduces the thickness of the contact disc andsubstantially eliminates the voids. A cross section of the repressedcontact disc III is illustrated in Figure and a perspective view thereofin Figure 6.

The depth of the degraphitized layer of the contact will depend upon thetemperature at which the sintering operation is carried out and thelength of time the piece is held at that temperature. Hence if adegraphitized layer of greater thickness is desired the time ofsintering will be correspondingly extended.

Figure 7 illustrates another method of producing a contact of the typedescribed. According to this method the surface of the pressed powderdisc 20, similar to disc l0 previously described, at which it is desiredto retain the graphite, is painted or otherwise coated with a protectivelayer 2| such as a paint formed from refractory material with a suitablebinder. The refractory paint protects one face of the disc from theoxidlzing atmosphere allowing the other surface of the disc to bedegraphitized. The refractory material may subsequently be removed.

While the invention has been described in its preferred form as appliedto silver graphite contact compositions, the invention also contemplatesother compositions for contact purposes and other uses. For example, amixture of copper powder with various amounts of graphite may be used;likewise a mixture of copper, silver and graphite powders is suitable.Other powder mixtures which are suitable, particularly for contactpurposes, are silver plus tungsten plus graphite, silver plus molybdenumplus graphite, copper plus tungsten plus graphite, silver plus nickelplus graphite, silver plus nickel plus cadmium plus graphite. Any ofthese metals may likewise be used with other forms of carbon instead ofgraphite as well as certain carbonaceous compounds, particularly thetype which decompose leaving a residual carbon deposit upon sinterlng.It will also be understood that while a composition of silver and 5%graphite is mentioned by way of illustration various other proportionsof the materials may likewise be used.

Although the invention has been described as applied to electriccontacts it is also contemplated that other types of face plates orsurfacing elements, such as bearings, clutch facings and the like comewithin the purview of the invention.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specific embodiments thereof, it isnot desired to be limited thereby but it is intended to cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. The method of making electric contacts which comprises mixinggraphite and contact metal powders, pressing the mixture into a body ofsuitable shape, coating the contact face of said body with a protectivelayer of refractory paint and then sintering said body in an oxidizingatmosphere, whereby to produce a sintered contact body having graphiteat and adjacent the contact face thereof and substantially devoid ofgraphite at the back face thereof.

2. The method of making a weldable silvergraphite electric contactelement, comprising coating 9. surface of said element with a refractorypaint and then oxidizing the graphite from the exposed surfaces of saidelement to thereby leave a readily weldable silver face substantiallyfree of graphite.

EARL I. LARSEN. EARL F. SWAZY.

